The wet-process for manufacture of phosphoric acid is described in KIRK-OTHMER "ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY" (1964) Vol. 9, page 87 et seq.; Vol. 15, page 261 et seq. Common impurities include sulfate, fluoride, chloride, iron, aluminum, vanadium, chromium, calcium, sodium and ammonium.
In view of the fact that specific details depend to a great extent on the original rock and the degree of purity desired, a detailed description of any one operation would not be typical of the industry. The prior art has in general considered it a necessity to employ a succession of precipitations followed by filtering or other forms of mechanical separation.
Thus, for example, gradual increase in pH by addition of soda ash can successively precipitate the phosphates of iron, aluminum and finally alkaline earths. According to some approaches, sodium silicate has been advocated to remove fluorine, or alternatively, superheated steam is blown through concentrated phosphoric acid to reduce the fluorine content. Iron and vanadium have been removed from the acid by addition of potassium ferrocyanide (A. V. SLACK - Phosphoric Acid (1968) part II, p 687 et seq)
Frequently, these prior-art methods have been both cumbersome in themselves and fraught with subsequent cumbersome complications. Thus, when precipitating agents are added as aqueous solutions, the acid is diluted and must be concentrated, sometimes repeatedly. The concentrations of phosphoric acid by evaporation must be carried out in corrosion-resistant equipment such as hard-lead vacuum evaporators or by blowing hot air or combustion gases at extremely high temperatures through carbon nozzles submerged in a shallow stream of acid flowing down a long trough.
Extractions by various solvents have also been attempted as means of carrying out individual purification steps. Such solvents have included, for example, alcohols, ethers, alkyl phosphates, amines and organosulfonic acids. However, to the best knowledge of present inventors, no systematic succession of extractions has been proposed which is aimed at successful preparation of a phosphoric acid product having extremely high purity. Regardless of the solvent chosen, it has not been possible to obtain with a simple extraction of wet-process phosphoric acid, a product sufficiently pure to take the place of purified thermal-process phosphoric acid in any of its numerous high-purity applications (Phosphorus and its compounds-VAN WAZER (1961) vol II p 1051 et seq)
In some instances, the early failures have been attributable to using solvents which are selective toward cations but much less selective toward anionic impurities which often include sulfuric, hydrofluoric, hydrochloric and fluosilicic acids, whose behavior in extraction is very similar to that of phosphoric acid itself.
It has been proposed to eliminate certain of these impurities by supplementary treatments with precipitating agents, during or after the treatment with solvent, but such treatments are relatively complicated, are rarely quantitative, and do not eliminate all the above mentioned impurities.
It has also been proposed to purify wet-process phosphoric acid by two successive extraction stages both using amines as solvents, but with such a procedure only a small part of the H.sub.3 PO.sub.4 is recoverable as pure aqueous extract (ROSE R.A. (MONSANTO) U.S. Pat. No. 3,865,439 - Aug. 15, 1971)